The cytogenetic aspects of ALL including the occurrence of sister chromatid exchanges (SCE) and their relation to the evolving cell populations as identified by chromosome markers will be studied. Prior studies have demonstrated an increase in SCE in PHA stimulated leukocyte and bone marrow cultures and in unstimulated bone marrow of untreated, newly diagnosed ALL. This project will use stimulated bone marrow and leukocyte cultures stained simultaneously for banding and SCE. Comparisons of distribution of SCEs among the human chromosomes and population changes during initial stages, remission and blastic crisis will be made.